The air conditioning systems of motor vehicles are circuits adduct a coolant and are formed by a plurality of components, comprising in particular a compressor, a condenser, a dryer tank, and expansion system, and an evaporator. All these components are connected to one another by means of tubular elements which have fixing elements and joining elements capable of ensuring fluid-tightness at their ends.
The constituent components of the air conditioning system are accommodated inside the engine compartment of the vehicle, with the compressor driven by the same driving shaft of the vehicle, while the other components are fixed to portions of the body. There are low pressure elements and high pressure elements in the air conditioning system. The latter may be subjected in use to coolant pressures of the order of 30 bars.
Freon gas named “R-134” has been used for a long time as coolant in cars. In order to obviate the polluting properties of such a gas, it is particularly important for a pipe intended to convey it to be substantially impermeable. Furthermore, low permeability is also desirable for the system to keep its functionality and efficiency over time.
However, international standards concerning the environment enforce seeking alternative solutions to Freon R-134 with a lower GWP (Global Warming Potential). Among these, the effectiveness of gas 1234 YS proposed by Honeywell and Dupont has been proven. In all cases, even when using a gas with lower GWP as a coolant, all elements, i.e. pipes and joints intended to convey it, must have the lowest possible permeability, in combination with satisfactory high-pressure mechanical properties, in particular after aging and substantially over the entire lifecycle of the motor vehicle.
In particular, car manufacturers require that the lines formed by pipes and joints intended for conveying coolant into the air conditioning system pass a plurality of experimental tests, e.g. hot burst tests for checking the mechanical features, strength tests to cyclic pressure variations, conveyed fluid permeability tests and chemicals resistance tests.
In automotive air conditioning systems, these requirements are generally met by using aluminum piping for conveying the coolant, at the ends of which brazed flanges and intermediate rubber pipes with bell-shaped joints or quick couplings moulded on the rubber itself are used, by possibly using such a metal in combination with multi-layer rubber pipes.
However, the general trend in the automotive field is to replace the metal or rubber piping where possible with equivalent structures made of plastic, so as to promote a reduction of manufacturing costs in addition to the total weight of the resulting air conditioning system, being thus beneficial for engine CO2 emissions by virtue of lower consumptions.
The systems are also provided with a sensor or other component which must be branch-connected, in particular screwed.
The need to keep the interfaces normally used in systems formed by metal or rubber piping arises when plastic pipes are used in an air conditioning circuit.
In particular, the tightening torque of the sensor is such to damage a thread made of thermoplastic material and cause coolant leakages.